1.Quadrophenia is my favorite Who album, no question.
2.Tommy
3. Who's Next
4. The Who Sell Out (I love the pirate radio format)
5. A Quick One.
If you add in live albums, Live At Leeds would knock everything below Tommy down a place.

At the moment my top 5 goes like this:
1. Pink Floyd
2. Queen
3. The Who
4. Led Zeppelin
5. Black Sabbath
(with The Beatles a very close 6th).
My top 20 bands generally do swithc places every once in a while, so this very well could change at any point.